The substances mostly used as propellants for aerosols are: Freon 11 (CCl.sub.3 F) and Freon 12 (CCl.sub.2 F.sub.2), which are commonly used in admixture with each other. There has been advanced the hypothesis that these substances, as a consequence of their accumulation in the atmosphere, may provoke the degradation of the stratospheric ozone.
Even though it could not be verified so far whether this hypothesis is well-founded or not, the above envisaged possible action is attributed to the decomposition of these substances under the impact of ultraviolet radiation, with the formation of chlorine radicals which, by a chain-reaction would degrade the O.sub.3 to O.sub.2.
On the contrary, it is considered that the hydrogen-containing fluorocarbons and chlorofluorocarbons cannot degrade stratospheric ozone. Such compounds may thus replace Freons 11 and 12 as aerosol propellants.
Even though numerous hydrogenated fluorocarbons and chlorofluorocarbons derived from methane and from ethane have boiling points quite suited for being used as propellants, they have not found application on a commercial scale as propellants because they do not possess the desired combination of chemical properties possessed by Freon 11 and 12, which enabled them to be used with success in a wide range of formulations for aerosols.
One of the basic chemical properties of aerosol propellants is their dissolving power, that is, their capacity to form a homogeneous liquid phase with the other components of the aerosol formulations, that is: the active ingredients, the solvents for said active ingredients, and the various auxiliary ingredients commonly used.
Only Freons 21 (CHCl.sub.2 F) and 11 have been considered good solvents, and this explains also why the most typical propellant composition consists of a component that is a good solvent (Freon 11) and of a component that raises the vapour tension (Freon 12).
Of interest is the publication by Paul A. Sanders ("Principles of Aerosol Technology," Van Nostrand Reinhold Company, New York, 1970, page 93), which reports the solvent power (expressed as the Kauri-butanol value) of some Freons, in a decreasing order of the solvent power:
______________________________________ Freon 21 CHCl.sub.2 F 102 Freon 11 CCl.sub.3 F 60 Freon 113 CCl.sub.2 F--CClF.sub.2 31 Freon 22 CHClF.sub.2 25 Freon 142b CH.sub.3 --CClF.sub.2 20 Freon 12 CCl.sub.2 F.sub.2 18 Freon 114 CClF.sub.2 --CClF.sub.2 12 Freon 152a CH.sub.3 --CHF.sub.2 11 ______________________________________
The Freons with the greater solvent power are in general those which, at equal number of carbon and hydrogen atoms contain more chlorine atoms. On the other hand, those with the highest content in chlorine show a certain toxicity. This is one example of the difficulty to conciliate the various requisites that the aerosol propellants must satisfy.
In spite of its satisfying performances, the Freon 11 plus Freon 12 mixture, besides the problem of the possible degrading of the stratospheric O.sub.3, brings with it a number of drawbacks. Freon 11 shows a certain toxicity. The mixture of the two Freons does not develop a satisfactory solvent power in certain applications, in particular, in those where the active ingredient consists of a resin as is the case of hair lacquers. In order to obtain a homogeneous lacquer formulation, it is necessary to use considerable quantities of the solvent of the resin, that is, usually anhydrous ethanol, with a corresponding burden of an increased cost, or revert to the addition of methylene chloride which is a good solvent of a low cost but which is relatively toxic. Moreover, Freons 11 and 12 have a relatively high molecular weight, wherefore, at equal weight, they deliver a smaller volume of gas in comparison to the propellants of lower molecular weight and thus have a lower yield in gas.
In certain aerosol formulations, there are also used saturated hydrocarbon propellants. The hydrocarbons most commonly used for this purpose are: propane, n-butane and iso-butane gases, possibly used in admixture with low-boiling liquid hydrocarbons, in particular, pentane and isopentanes.
These hydrocarbons have a low cost and a low molecular weight, but they have two serious drawbacks with regard to their use in aerosol compositions. The first drawback is their high inflammability, while the second is their low solvent power.
In consideration of these drawbacks, the hydrocarbon propellants are almost exclusively used in formulations containing an aqueous phase which contains the active ingredients in the state of an emulsion or suspension.
During the use of the spraying bottles, the presence of water in the aerosol jet reduces the risk due to the inflammability of the hydrocarbons.